Science 7–10 · Years 9–10

Neutralisation: how the pH changes as acid is added to an alkali

Reactions

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Adding an acid to an alkali neutralises it: the pH falls only slowly at first, then very sharply at the point where the alkali has just been used up, and an indicator shows that point as a sudden colour change.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

sodium hydroxide and hydrochloric acid solutions irritate eyes and skin
methyl orange is harmful if swallowed
glass burette and pipette

Controls

eye protection
pipette filler only, never mouth
burette filled below eye level using a funnel, then the funnel removed
spills wiped at once

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.

What you need

sodium hydroxide 0.100 mol/L, 25.0 mL measured with a pipette and pipette filler
hydrochloric acid 0.100 mol/L in a 50 mL burette
conical flask, white tile, phenolphthalein or methyl orange indicator, funnel, pH meter or probe
evaporating basin for the crystallisation follow-on

How to do it

Pipette 25.0 mL of the alkali into the flask, add 3 drops of indicator, put the pH probe in the liquid and stand the flask on the white tile under the burette.
Run in the acid 1.0 mL at a time with swirling, recording the pH and the colour after each addition; from 23 mL add 0.1 mL (about two drops) at a time until 27 mL, then 1.0 mL at a time to 30 mL.
Plot pH against the volume of acid added and mark the volume at which the indicator changed colour.
Repeat with fresh alkali, running the acid in quickly to 23 mL and then drop by drop, and check that the colour changes at the same volume as in the first run.
Follow-on: add that volume of acid to fresh alkali without indicator and evaporate the solution to obtain sodium chloride crystals; write the word equation and the balanced equation with states.

What you should see

The pH starts at 13.0 and falls only slowly: it is 12.0 after 20.0 mL of acid and still 10.3 at 24.9 mL. It then drops to 7.0 at 25.0 mL and 3.7 at 25.1 mL, more than 6 pH units in 0.2 mL, and levels off near 2 by 30 mL. With the acid and the alkali at the same concentration the drop comes when the acid volume equals the alkali volume, 25.0 mL, and the indicator changes colour within it. The learner knows it worked when the graph shows a slow fall, a near-vertical drop and a flat end, the indicator changes colour within the steep drop, and the repeat run changes colour within 0.2 mL of the first.

What changes

What you change: volume of acid added
What you measure: pH and indicator colour
What you keep the same: alkali volume and concentration
acid concentration
indicator amount
temperature

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The pH changes evenly as acid is added.
Neutral means all the acid and alkali have disappeared.
A stronger colour means a more concentrated acid.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC5-RXN-01 SC5-WS-01
Australian Curriculum v9AC9S9U07AC9S10U07AC9S10I03AC9S9I03

Sources

The pages the author read to write this activity.

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